1) Field of the Invention
The present invention relates to an acrylic rubber composition, and more particularly to an acrylic rubber composition having a property to prevent fixation to a metal, etc.
2) Description of the Prior Art
Acrylic rubber is used as molding materials for sealing rubber such as packings, O-ring, etc. and has such a tendency that the rubber is fixed to materials in direct contact with the rubber, for example, metals such as cast iron, aluminum, stainless steel, soft steel, etc., or resins such as polyamide resin, phenol resin, etc., thereby lowering the sealing effect. Such a tendency is often observed particularly in case of acrylic rubber having cross-linkable groups such as reactive halogen, epoxy group, etc. in the acrylic rubber molecules.
The term "fixation or fixing" used herein designates a state of sealing rubber, which cannot be peeled off from a material, especially cast iron or mild steel, in contact with the sealing rubber by strong adhesion due to reaction therebetween, when the sealing rubber and the material in contact therewith are compressed by about 10 to about 40% and left standing in air or working oil at a predetermined temperature for a predetermined time.
Various methods have been so far proposed, as given below, to solve the problem of fixation, but have not been found successful:
(1) A method of adding a relatively large amount of a plasticizer, etc. to the rubber, thereby bleeding the plasticizer, etc. from the rubber to prevent any direct contact with a mating material. The method is economically best, but the physical properties of the rubber are largely lowered by the bleeding. PA1 (2) A method of depositing a solid lubricant onto the rubber. The method suffers from easy release of a solid lubricant such as graphite, molybdenum disulfide, etc. from the rubber, resulting in unstable properties and contamination of working oil with the released solid lubricant to form foreign matters. PA1 (3) A method of coating a sealing material with a composition of a rubber-based binder and a solid lubricant as main ingredients, dispersed in a solvent. The method usually suffers from a poor adhesion to the sealing material and has no satisfactory effect on the prevention of fixation. PA1 (C.sub.1) vinyl monomer having an epoxy group, PA1 (C.sub.2) vinyl monomer having a carboxyl group, PA1 (C.sub.3) vinyl monomer having a reactive halogen, PA1 (C.sub.4) vinyl monomer having a hydroxyl group, PA1 (C.sub.5) vinyl monomer having an amide group, and PA1 (C.sub.6) dienic monomer. PA1 (C.sub.1) For the vinyl monomer having an epoxy group including, for example, allylglycidyl ether, glycidyl acrylate, glycidyl methacrylate, etc., such cross-linking agents as polyamines, for example, diethylenetriamine, metaphenylenediamine, etc.; polycarboxylic acids, for example, adipic acid, etc.; acid anhydrides, for example, pyromellitic anhydride, maleic anhydride, etc.; and polyamides; sulfonamides, etc. can be used. PA1 (C.sub.2) For the vinyl monomer having a carboxyl group including, for example, acrylic acid, methacrylic acid, itaconic acid etc., such cross-linking agents as polyepoxides, for example, ethyleneglycol diglycidyl ether 1,6-hexanediol diglycidyl ether, etc.; and polyols, for example, 1,4-butanediol, 1,1,1-trimethylolpropane, etc. can be used. Cross-linking can be carried out only by heating without using these cross-linking agents. PA1 (C.sub.3) For the vinyl monomer having a reactive halogen including, for example, 2-chloroethylvinyl ether, etc., such cross-linking agents as polyamines, for example, diethylenetriamine, triethylene-tetramine, etc.; and polycarbamates, for example, hexamethylenediamine carbamate, etc. can be used. PA1 (C.sub.4) For the vinyl monomer having a hydroxyl group including, for example, hydroxyalkyl methacrylate, hydroxyalkoxy acrylate, N-methylolacrylic acid amide, etc., such cross-linking agents as polyisocyanates, for example, hexamethylene diisocyanate, tolylene diisocyanate, etc.; polycarboxylic acids, for example, adipic acid, etc.; and alkoxymethylmelamines, for example, methoxymethylmelamine, etc. can be used. PA1 (C.sub.5) For the vinyl monomer having an amido group including, for example, acryl amide, methacryl amide, etc. such cross-linking agents as aminoformaldehyde, etc. can be used. Cross-linking can be carried out only by heating. PA1 (C.sub.6) For the dienic monomers including, for example, divinyl benzene, isoprene, pentadiene, vinylcyclohexene, chloroprene, butadiene, methylbutadiene, cyclopentadiene, methylpentadiene, ethyleneglycol diacrylate, propyleneglycol diacrylate, ethyleneglycol dimethacrylate, propyleneglycol dimethacrylate, etc., such cross-linking agents as sulfur; organic peroxides, for example, benzoyl peroxide, dicumyl peroxide, etc.; azo compounds, for example, azobisisobutyronitrile, etc.; divinyl benzene; triallyl cyanurate, triallyl isocyanurate, etc. can be used. Cross-linking can be carried out only by heating without using these cross-linking agents.